https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312
--- Comment #7 from Kostas Sotiropoulos <kosotiro at yahoo dot gr> --- (In reply to Andrew Pinski from comment #5) > (In reply to Kostas Sotiropoulos from comment #4) > > Any comments on my questions? > > Yes go read the c standard about prompting to int here. I had checked the standard from the following link: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf I suppose that you mean the following two sections that cover our case: 6.5.16.2 A compound assignment of the form E1 op=E2 is equivalent to the simple assignment expression E1 = E1 op (E2), except that the lvalue E1 is evaluated only once, and with respect to an indeterminately-sequenced function call, the operation of a compound assignment is a single evaluation. 6.3.1.1 Boolean, characters, and integers 2 The following may be used in an expression wherever an int or unsigned int may be used: — An object or expression with an integer type (other than int or unsigned int) whose integer conversion rank is less than or equal to the rank of int and unsigned int. — A bit-field of type _Bool, int, signed int, or unsigned int. If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions. All other types are unchanged by the integer promotions. So considering the above two sections, if we even cast i += (unsigned char) MACRO then this would be equal to i = i + (unsigned char) MACRO. But, what if I want to avoid this warning then the only way to get rid of it is the following way? i = (unsigned char) (i + MACRO). But we will lose on code optimization be this way, do not we? Especially when we are talking about Embedded Systems software. Please note that the same code snippet passes through clang compiler (clang version 3.8.1-24) without any warning when -Werror=conversion is used during compilation. I suppose that even this compiler follows C standard. In my opinion such kind of warnings are false positives that should not be reported from gcc compiler too. Maybe a change of this kind on gcc compiler opens the Pandora's Box for other things but someone should not avoid of defining MACROS and use them on compound statements (+=, -=, *= etc) and sacrifice code optimization by expanding them. Waiting for your comments.
